This is a proposal to continue examination of the ontogeny of the neural circuitry underlying the processing of species-typical vocalizations in the mammalian auditory system, at the levels of the inferior colliculus and auditory cortex. Studies of language acquisition in humans and song learning in birds show that genetic instruction directs attention to behaviorally relevant vocalizations, while experience during a critical period is necessary for maturation. During this sensitive period, the development of appropriate spectrotemporal filters is thought to be crucial for normal maturation of the perception and production of vocalizations. During the previous grant period, the pallid bat auditory system was used to document the time course of the development of neuronal selectivity for the echolocation pulse, an FM sweep common in the vocalizations of many species, including human language. These studies also revealed some of the inhibitory mechanisms that shape this selectivity. They also show that auditory cortex, rather than simply inheriting response selectivity, further tunes this selectivity. Aim 1 addresses the finding that muted bat pups raised with echolocating adults did not develop normally, suggesting that pups must actively produce echolocation pulses to develop adult-like neuronal selectivity. Aim 2 will use the blockade of GABAergic receptors to reveal the nature of the inhibition laid down during development at both the collicular and cortical levels. Aim 3 extends the finding that, during development, neurons lacking adult-like inhibitory response properties can nonetheless express response selectivity shaped by facilitatory mechanisms. These mechanisms will be examined. Aim 4 will develop neuroanatomical measures of maturation. The thalamocortical projections of the pallid bat auditory system are unusual;the pathways serving echolocation and passive hearing are routed through different subdivisions of the medial geniculate body. During ontogeny, the two thalamocortical pathways overlap. The time course for pathway segregation will be examined. In summary, the pronounced physiological and anatomical specializations of the pallid bat auditory system offer an opportunity to broadly address the role of experience in influencing the maturation of the neural circuitry serving communication in the mammalian auditory system.